Phototoxic target lipid model for predicting the toxicity of polycyclic aromatic hydrocarbons and petroleum to aquatic life

Department: University of Delaware, Department of Civil and Environmental Engineering

Publisher: University of Delaware

Date Issued: 2016

Abstract: The objective of this doctoral dissertation is to develop a model to predict the
phototoxicity of petroleum and petroleum components to aquatic organisms.
Petroleum contains polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs and
heterocyclic PAHs some of which absorb light in the ultraviolet light (UV) and visible
(VIS) regions. The result is increased photo-enhanced toxicity, by a factor of two to
greater than 1000 in the presence of light. -- The PAHs in petroleum differ in their properties, such as octanol-water
partitioning coefficients and molar absorption spectra, and each may exhibit
phototoxicity. It is inefficient and impractical to conduct toxicity tests on all the
chemicals and all the organisms of concern. Even if the testing was undertaken, it is not
clear how to interpret the results and use them for phototoxic risk assessments where
light conditions and time of exposure vary. Accordingly, there has been a considerable
effort expended to develop models to predict the phototoxicity of PAHs to the aquatic
organisms. In each of the previous modeling frameworks various combination of the
underlying factors in phototoxicity were incorporated to varying degrees. However, no
model included all elements in a unified modeling framework such that the model can
be applicable to all PAHs, PAH mixtures, organisms, and light exposure conditions. -- In this dissertation, a phototoxic target lipid model (PTLM) is developed to
predict phototoxicity of single PAHs measured either as median lethal concentration
(LC50) at a fixed duration of exposure or median lethal time (LT50) at a fixed
concentration. The model accounts for differences in the physical and chemical
properties of PAHs and test species sensitivities, as well as variations in light
characteristics, such as length of exposure, and the light source irradiance spectrum
and intensity. The PTLM is based on the narcotic target lipid model (NTLM) of PAHs.
Both models rely on the assumption that mortality occurs when the toxicant
concentration in the target lipid of the organism reaches a threshold concentration. The
model is calibrated using 333 observations of LC50s and LT50s for 20 individual PAHs,
15 test species, and various UV light exposure conditions and times ranging from 1
hour to 100 hours. The LC50 concentrations range from less than 0.1 to greater that 104
μg/L. The model has two fitting parameters that are shown to be constant across PAHs
and organisms. The compound specific parameters incorporated in the PTLM are the
octanol-water partition coefficient and molar absorption coefficient. The critical target
lipid body burden is the only organism specific parameter. The root mean square error
(RMSE) of prediction for log(LC50) and log(LT50) are 0.473 and 0.382, respectively.
Other phototoxic components of petroleum include alkylated PAHs (APAHs) and
benzothiophenes. The PTLM is validated by predicting the observed phototoxic LT50
and LC50 of those chemicals exposed to four different species under different light
conditions with RMSE = 0.478. The results support the PTLM capability to predict the
phototoxicity of single PAHs for organisms with a wide range of sensitivity and for
various light exposure conditions. -- Modeling the phototoxicity of mixtures is accomplished by using the toxic unit
(TU) approach and TU additivity. The model is validated by predicting the
phototoxicity of the binary and ternary mixtures of three PAHs, pyrene, anthracene,
and fluoranthene exposed to Americamysis bahia and Menidia beryllina. The
comparison between the observed and predicted phototoxicity for the mixtures results
in RMSE = 0.274. -- The PTLM is applied to predict petroleum phototoxicity of the water
accommodated fraction for three field collected oil samples, MASS (neat oil), CTC
(moderately weathered oil), and Juniper (heavily weathered oil) exposed to four
aquatic species indigenous to the Gulf of Mexico, M. beryllina, A. bahia, Cyprinodon
variegatus, and Fundulus grandis using natural or simulated solar radiation. For cases
in which no phototoxicity was observed, the PTLM predictions are correct in over
70% of the cases (10 out of 14 predictions). When toxicity was observed the RMSE =
0.321.